Tweaks to DragAndDropPermissions behavior

No longer release permissions in finalize(), so that
apps do not have to maintain a reference to the
DragAndDropPermissions object.

Also make it parcelable, so that permission instances can be
retained across activity instances so that they can be
manually released.

Bug: 29162822

Change-Id: Ie604dd3e83ee45a8665d743449b91857dd54e896